The present invention relates to a digital sound generating system which is capable of, in addition to forming a tone waveform (first waveform data), reproducing or recording a waveform of a human voice or sound of music lasting for a relatively long time (second waveform data).
Today, in many fields, efforts are being made to enjoy the benefits of the so-called "multimedia" for enriching types of information to be provided to users and forms of receiving and transmitting information to users. Various types of equipment, such as personal computers, incorporating therein a PCM tone generator were provided in the past, but in recent years, equipment provided with functions to reproduce or record human voices or sounds of an actually performed music piece sampled from the outside (hereinafter called "vocal sounds or the like" or "second waveform") as well as electronic sounds (hereinafter called "tones" or "first waveform") have been developed. Because vocal sounds or the like have generally much longer duration than tones, personal computers and similar equipment have been developed which contain dedicated hardware, such as a PCM sound reproducing and/or recording device, for reproduction and/or recording of such vocal sounds or the like.
FIG. 5 is a block diagram showing an exemplary structure of a conventionally-known PCM sound reproducing device, which comprises a CPU interface 21, a timing control section 22, a FIFO memory 23, a digital-to-analog (D/A) converter 24 and an analog filter 25. The PCM sound reproducing device is incorporated in a personal computer or other system having a large-capacity storage device such as a hard disk device (not shown). In the large-capacity storage device of the system, there are prestored PCM sound data made by sampling and coding a continuous waveform, as of a human voice (vocal sound), having relatively long duration or sounding time. In reproduction of the vocal sound, the PCM sound data are read out from the large-capacity storage device and transferred to the PCM sound reproducing device under the control of a CPU (not shown).
The CPU interface 21 is a means for receiving and delivering information between the system's CPU and the PCM sound reproducing device, and the PCM sound data are introduced via this interface 21 into the PCM sound reproducing device and accumulated in the FIFO memory 23.
The timing control section 22 controls respective operational timing of the various components of the PCM sound reproducing device. In reproducing the vocal sound, the timing control section 22 sets a sampling frequency at which the PCM sound data are to be reproduced (reproduction sampling frequency) in accordance with instructions given from the CPU, and generates sampling clock pulses at the thus-set reproduction sampling frequency. In response to the sampling clock pulses, the PCM sound data are sequentially retrieved, one by one, from the FIFO memory 23 and converted by the D/A converter 24 into analog representation.
In normal cases, the reproduction sampling frequency of the PCM sound data is set to be substantially lower than the double of the highest audio frequency, so that the analog signals output from the D/A converter 24 will contain aliasing noise. For this reason, a high-performance analog filter 25 having a sharp cut-off characteristic is provided at a stage following the D/A converter 24, and each analog signal output from the D/A converter 24 is passed through this filter 25 so that the analog signal without aliasing noise is supplied to a sound output device such as a speaker. To effectively remove the aliasing noise, it is preferred that the analog filter 25 of a considerably large scale be used. In many cases, a switched capacitor filter is used as the analog filter 25.
Once the number of the PCM sound data in the FIFO memory 23 has been reduced to a predetermined value by the sound data being sequentially retrieved from the memory 23, the timing control section 22 sends, via the CPU interface 21, a request to the CPU to further supply succeeding PCM sound data. In response to the request, the succeeding PCM sound data are sequentially read out from the large-capacity storage device and introduced into the reproducing device via the CPU interface 21. The introduced PCM sound data are sequentially accumulated in the FIFO memory 23 under the control of the timing control section 22.
The thus-accumulated PCM sound data are then sequentially reproduced at a predetermined rate, during which time further succeeding PCM data are supplied to the FIFO memory 23, so as to reproduce the continuous waveform of the vocal sound lasting for a long time.
FIG. 6 is a block diagram showing an exemplary structure of a conventionally-known PCM sound recording device, which comprises a CPU interface 31, a timing control section 32, a FIFO memory 33, an analog-to-digital (A/D) converter 34 and an analog filter 35. As in the above-mentioned PCM sound reproducing device, the PCM sound recording device is incorporated in a personal computer or other system having a large-capacity storage device such as a hard disk device (not shown).
The timing control section 32, prior to recording a vocal sound, sets a sampling frequency at which a continuous waveform of the vocal sound is to be sampled (recording sampling frequency) in accordance with instructions given from the CPU, and generates sampling clock pulses at the thus-set recording sampling frequency. Each analog signal representing the waveform is passed through the analog filter 35 so that high-frequency components are removed from the signal via the filter 35, and then the analog signal is supplied to the A/D converter 34. In the A/D converter 34, the analog signal is sampled in accordance with the sampling clock pulse and converted into PCM sound data.
In the known PCM sound recording device, the analog filter 35 eliminates the high-frequency components from the analog signal in advance in such a manner that the PCM sound data from the A/D converter 34 does not contain unwanted aliasing noise. For this reason, the analog filter 35 needs to be a high-performance filter having a sharp cut-off characteristic, as in the above-mentioned PCM sound reproducing device. Further, because the recording sampling frequency of the A/D converter 34 is optionally changed in this recording device, the analog filter 35 is of a type whose cut-off frequency is adjustable depending on a change in the recording sampling frequency.
Each PCM sound data output from the A/D converter 34 is accumulated in the FIFO memory 33. Once a predetermined quantity of the PCM sound data have been accumulated in the FIFO memory 33, the timing control section 32 sends, via the CPU interface 31, a request to the CPU to store the accumulated PCM sound data. When the CPU accepts the request, the PCM sound data accumulated in the FIFO memory are read out under the control of the timing control section 32 and output via the CPU interface 31 to the large-capacity storage device for storage therein.
In the above-mentioned manner, analog signals of vocal sounds or the like are converted into PCM sound data and accumulated in the FIFO memory 33, and each time the predetermined quantity of the PCM sound data have been accumulated, they are read out from the FIFO memory 33 and stored into the large-capacity storage device.
However, in the known personal computer or other system, it was necessary that the PCM sound reproducing or recording device be provided separately from a tone generator, in order to reproduce or record a long-lasting continuous waveform of a vocal sound or the like in addition to generating a tone. Further, the PCM sound reproducing or recording device required a high-performance analog filter in order to obviate aliasing noise, and in cases where the reproduction or recording sampling frequency was to be changed, the analog filter itself had to be of large scale because it was necessary to change the characteristic of the analog filter. As a result, the known system required high overall costs.